1. Field of the Invention
This invention relates to liquid level sensors, and more particularly to liquid level sensors for measurement of a liquid level in a container and for controlling a fluid inlet to the container in response to the liquid level therein.
There are many situations where it is desirable to measure and control the liquid level in a container. One such instance which has given rise to the need for the present invention is in the slurry transportation method of removing coal from a coal mine. With the availability of high speed, automatic mining machines and the development of long wall mining techniques, the limiting factor in many coal mining operations has become the ability to transport the coal particles from the mine to the surface. The use of the conventional railway cars and conveyors in many cases cannot keep up with the progress of the coal-cutting machinery, and as a result the mining machines are only utilized for a fraction of their capacity. In an effort to remove this limitation from coal mining operations, slurry transportation systems have been developed in which the coal particles are slurried with a circulating liquid and pumped by high capacity pumps from the interior of the mine to the surface. Slurry-handling systems are presently available which provide the ability to rapidly remove large amounts of coal particles from a mine without the necessity of conveyors and railway cars as were needed using conventional mining techniques. However, even with these high capacity slurry-handling systems, certain problems have arisen due to the space limitations inherent in most mining operations. The volume of slurry required to handle a modern mining operation is of the order of 15,000 or more liters per minute of slurry. Such a high volume slurry operation normally would call for a very large volume feed hopper to smooth out the operation and to protect against emptying of the feed hopper with possible resultant pump damage and other operating difficulties. However, due to the space limitations in an underground mine, the feed hopper size is severely restricted, and this necessitates a particularly sensitive and rapid response for the inlet control to the slurry hopper. Due to the high flow rates, the slurry hopper can be pumped dry in a very short time, sometimes even as quickly as a fraction of a second, such that the need for precise and immediate control is apparent.
The present invention provides a reliable means for accurately and rapidly controlling the inlet flow rate to a slurry hopper of the type used in slurry transportation mining systems.
2. Description of the Prior Art
Numerous float-operated level control devices are available. U.S. Pat. Nos. 3,563,091 and 3,866,470 are exemplary of float-operated level indicators. U.S. Pat. Nos. 3,543,580 and 3,805,613 are exemplary of level indicators comprising flexible members having buoyant means at a free end and internal means responsive to the elevation of the float member to indicate the liquid level in a tank.
The high liquid flow rate and the large volume of coal particles handled by a slurry hopper for a coal transportation system necessitates a separate compartment for the level control to protect the control means for damage due to the coal particles being moved rapidly through the hopper. U.S. Pat. No. 2,564,305 shows a buoyancy responsive device attached to a tank in a separate compartment for protection of the device.
Previously available measurement devices of the types shown in the above-discussed patents have not been suitable for the control of a slurry hopper in a coal slurry transport system. Some success has been had in the past utilizing a sensitive differential pressure cell connected to a protected compartment in fluid communication with a coal slurry hopper. However, even this has been unsatisfactory in that the conduit to the differential pressure cell has tended to plug with coal particles with resultant interference with its operation. Although the use of a pressure transducer such as a sensitive differential pressure cell has been better than alternative mechanical or electrical linkage devices, there has still been a need for an improved level sensor and control system. Such an improved level sensor and control system is provided by the present invention.